1. Field of the Invention
The invention relates to a process for preparing vinyl chlorosilanes by non-catalyzed, thermal reaction of chlorosilanes with vinyl chloride using a reactor combination of a ring-gap reactor equipped with an optionally rotating displacement body and a downstream reactor.
2. Discussion of the Background
Vinyltrichlorosilane is a valuable intermediate which, owing to its four reactive groups, is suitable for many applications. For example, it is used in the sizing of glass fibers and in the manufacture of cable materials.
DE-C 936 445, DE-A 22 10 189 and, in particular, DE-C 20 02 258 disclose that upon passing vinyl chloride/chlorosilane mixtures such as, for example, vinyl chloride/trichlorosilane mixtures, through appropriately-heated, empty ceramic, glass or iron tubes, industrially acceptable yields of vinyl chlorosilanes are achieved. The reaction proceeds purely thermally, i.e., without a catalyst. In the reaction of trichlorosilane with vinyl chloride, the following main reaction proceeds:
Main reaction: EQU C.sub.2 H.sub.3 Cl+SiHCl.sub.3.revreaction.C.sub.2 H.sub.3 SiCl.sub.3 +HCl
In addition to the main equilibrium reaction, there are the following side reactions and secondary reactions:
C.sub.2 H.sub.3 Cl+SiHCl.sub.3.fwdarw.SiCl.sub.4 +C.sub.2 H.sub.4 EQU 4 SiHCl.sub.3.fwdarw.3 SiCl.sub.4 +Si+2 H.sub.2 EQU C.sub.2 H.sub.3 Cl.fwdarw.C.sub.2 H.sub.2 +HCl EQU C.sub.2 H.sub.2.fwdarw.2 C+H.sub.2 EQU C.sub.2 H.sub.4 +SiHCl.sub.3 --.fwdarw.C.sub.2 H.sub.5 SiCl.sub.3 EQU C.sub.2 H.sub.4 +SiHCl.sub.3 --.fwdarw.C.sub.2 H.sub.3 SiCl.sub.3 +H.sub.2 EQU C.sub.2 H.sub.3 SiCl.sub.3 +SiHCl.sub.3.fwdarw.Cl.sub.3 SiC.sub.2 H.sub.4 SiCl.sub.3
Despite these side reactions and secondary reactions, according to DE 40 01 820 A1, the selectivity for vinyltrichlorosilane, based on the vinyl chloride used in deficient amounts, depending on the ratio of the starting materials and the degree of conversion, is from 50 to 98% in the case of tubes 122 to 150 cm in length and 25 to 35 mm in diameter, residence times from 0.2 to 20 seconds, reaction temperatures from 400 to 750.degree. C. and pressures from 1 to 3 bar. However, the reactor output (or capacity) of the reactors in this case is only from 0.8 to 3.2 metric tons of vinyltrichlorosilane/month. Selectivity and degree of conversion are inversely proportional to one another; the reactor output passes through a maximum as a function of the degree of conversion. A high selectivity is therefore accompanied with an unsatisfactory and economically unacceptable reactor output at a low degree of conversion. According to DE-A 20 02 258, although increasing the tube diameter up to 50 mm produces an increase in reactor output in proportion to the greater reactor volume, at still greater diameters, the specific reactor output, based on the reactor volume, decreases. It is therefore impossible to increase the space-time yield of vinyltrichlorosilane, or even only maintain it, by increasing the reactor tube diameter to above 50 mm.
An improved process for preparing vinyl chlorosilanes by reacting is chlorosilanes with vinyl chloride is, according to DE 40 01 820 A1, carried out in a ring-gap reactor which has a heatable reaction tube having an internal diameter d.sub.1 and in the interior of which is situated a displacement body having an outer diameter d.sub.2, and which extends over the entire length of the reaction tube, is axially and symmetrically disposed within the reaction tube and which may optionally rotate. The relationship d.sub.1 =d.sub.2 +2a applies here, where a is generally at least 1 cm and is always &lt;5 cm. If the displacement body is substantially shorter than the reaction tube, the yield is decreased. This finding corresponds to the teaching of the three abovementioned publications, according to which, in the case of empty tube reactors, the yield falls if the tube diameter exceeds 5 cm. According to DE 40 16 021 A1, the capacity of the reactor and the space-time yield of the process using a ring-gap reactor can be further increased if the reaction components are preheated to 120 to 400.degree. C. prior to entering the reactor. However, even using this measure, much of the reactor volume is still lost in that the reaction components are heated to approximately 550.degree. C., in order that they react adiabatically in the remaining reactor volume.